For RAID 1:
Old drives: (HGST 1TB)
New drives
Thought the old ones were broken, the fact was then.....the cable was loosened. Anyway, the old drives started to show reallocated sectors for a while, and are definitely dying (when using more than 500GB there will be error), it's a good time to replace them I suppose.
For the new drives, transferring video from USB3 Seagate harddisk ---> RAID 1 of new drives is as fast as 180MB/s.